Dovetail

Introduction to Dovetail

Rationale for an overhaul

Dual kernel systems such as Cobalt need Linux to exhibit a high-priority
execution stage, for running out of band co-kernel activities. This is
the purpose of the I-pipe layer, patched in the mainline kernel.

In order to keep pace with mainline Linux development, this mechanism
should be simple, and properly integrated as a plain Linux feature,
not sideways. In other words, it must be possible to maintain it with
common kernel development knowledge, at a fraction of the engineering
and maintenance cost native preemption requires. For several reasons,
the I-pipe does not qualify.

As a matter of fact, maintaining the I-pipe proved to be difficult
over the years as changes to the mainline kernel regularly caused
non-trivial code conflicts, sometimes nasty regressions to us
downstream. Although the concept of interrupt pipelining proved to be
correct by delivering short response time with reasonably limited
changes to the original kernel, the way this mechanism is currently
integrated into the mainline code shows its age.

Issues to address

The I-pipe is composed of two major parts: the interrupt pipeline
mechanism and the co-kernel interface for sharing control of Linux
tasks between the regular kernel and Xenomai.

There are too many sideways introduced by the interrupt pipeline
machinery into mainline’s generic IRQ core and irqchip drivers which -
in addition to lowering the odds of conflicts with mainline - do not
follow the original logic, which in turn leads to restrictions and
fragilities, among which:

Several changes introduced in the interrupt flow handlers by the
I-pipe do not abide by the original locking scheme protecting the
IRQ descriptor logic. A direct consequence of this is that every
irqchip driver has to be fixed up / converted for reinstating some
serialization (e.g. hard spinlocks or interrupt disabling).

In a multi-processor system, the I-pipe bypasses the original
serialization logic of device IRQs (i.e. IRQD_IRQ_INPROGRESS),
which guarantees that each event cause is handled by a single CPU
only. As a result of this shortcoming, those IRQs need to be
serialized by Xenomai via custom spinlocks when handling them in
real-time mode.

The I-pipe cannot share a parent interrupt line between multiple
interrupt controllers, which is a problem for some devices we may
want to use in real-time mode (e.g. Intel pinctrl/GPIO).

The EOI signal is not properly handled for specific interrupt
controllers (i.e. the IRQCHIP_EOI_IF_HANDLED flag is not honored).

Lockdep is not available to most I-pipe ports, because interrupt
pipelining breaks the lock validator.

Keeping mainline’s IRQs off (f)tracing support requires invasive
changes in the assembly code, which is prone to merge conflicts
and regressions.

The I-pipe provides its own function tracer based on the Ftrace
engine, but still duplicating some of its features while
restricting others.

Context tracking is incompatible with the I-pipe
(CONFIG_CONTEXT_TRACKING) which forces it off.

Generally speaking, the interrupt pipeline logic needs a complete
overhaul for a better integration into the generic IRQ core, so that
the former becomes a regular feature of the latter, instead of
bypassing it in ugly ways in several occasions.

The co-kernel interface can be improved too in several aspects, such
as:

providing a simpler way to hand over the timer hardware to the
co-kernel. The current ipipe_timer abstraction mostly works
sideways to mainline’s clock chip one, which leads to code
duplication.